Reading CSV files

• CSVImport::__construct(): Initialise a CSVImport instance.
• CSVImport::readFields(): Read the list of field names from a CSV file.
• CSVImport::setFields(): Specify an explicit column-to-field mapping.
• CSVImport::rewind(): Move the file pointer back to the beginning of the file.
• CSVImport::rowFlat(): Read a row from the CSV file without mapping columns to fields.
• CSVImport::row(): Read a row from the CSV file.